Recent trends in digital comnrunications are opening h;ommercial applications to Code Diviqion Multiple Access (CDMA). The paper presents an analytical model for the effects of tiodinearities and mobile channel on the performance of synchronous QPsK ivi5;icsn ~u~t~p l e -~c c~s s (CDh4.A) systems. The combined eEect of both, the travelling wave tube (TWT) power amplifier exhibits PiM-to-AM an M-to-PM conversion, down-link thermal noise and the multipath fading mobile channel are taken into consid eration in the analysis. System performance in terms of the error probability is explored A key element determining the satellite network complexity and eXiciency is the multiple access scheme selected Considering the environmental conditions, the optimal access scheme s~~u~~ provide the following [l] cknt use o f the available transponder power and bandwidth for single or multiple beam satellites * Resistance to hostile propagation conditions like multipath fading and shadowing * Adaptability to highly variable trafic patterns. * Flexibility to different applications * Adaptability to different network configurations * Compliance with power flux density requirements for user terminals providing minimum R.F interference to other services.'Then: has been increasing interest recently in using direct sequence spread spectrum (DS-SS) e division m~~~i p~e access (CDMA) for corrmercial applications IDS-CDiteA. is propo Key Teatures of C D M are the ~~e i l -k~o w n robustness to narrow-band interference and to frequency selective multipath fading, secure communisation, the low power flux density emission and the possibili to exploit diversity reception [2]. The propagation channei can be classified either uns owed or shadowed channel The aim of this work is to study and test the link perfomlance under the joint effect of both, the transponder (TWTA) nonlinearity, the multipath fading mobile channel and the down-link AWGN. For this purpose, an analytical and simulation were performed for DS-CDMNQPSK for satellite [ 11 and for indoor/terrestrial [ 21, [3] M e r this introduction, section 2 describes DS-CDMA system model. The performance analysis is presented in section 3, Section 4 is concerned with mobile fading channel. A summary and concluding remarks are finally reported in section 5. DS-CDMA SYSTEM MODELThe system model is depicted in Fig. 1. The current approach to dealing with the near-far problem in direct-sequence code-division multiple-access (DS-CDMA) communication systems is to use transmitter power control. The input signal S(t) consists o f a sum of synchronous CDMA.The nonlinear amplifier can be seen as nonlinear pipe, which is especially well known in the satellite comunications. We hrther assume the down-link is an Additive White Gaussian Noise (AWGN) channel. This system model can be viewed as a satellite system with high uplink power which leads to omission of uplink noise.
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